1. Field of the Invention
The present invention relates to a transmission apparatus used for transmitting electric signals, optical signals, small power, etc. between a rotary body and a fixed body.
2. Description of the Related Art
FIG. 1 shows a transmission apparatus between a rotary body and a fixed body meant to be improved by the present invention. This transmission apparatus is provided at its center with an inside case 1 having an inner tubular portion 3 of a hollow cylindrical shape and an outside case 5 having an external tubular portion 7 disposed coaxially around the outer circumference of the inner tubular portion 3. The inside case 1 and the outside case 5 are joined together so as to be relatively rotatable. In the space formed between the inside case 1 and the outside case 5, one flat cable 9 and three dummy cables 11 are accommodated wound upon each other to form winding direction inversion portions 9a and 11a in an intermediate portion in the longitudinal direction.
The inner ends of the flat cable 9 and the dummy cables 11 are affixed to the inner tubular portion 3 and the outer ends to the outer tubular portion 7. The winding direction inversion portions 9a and 11a of the flat cable 9 and the dummy cables 11 are disposed at substantially equal intervals in the circumferential direction.
By the use of the above-mentioned construction, the inside case 1 and the outside case 5 can be rotated relatively within the rotational limitation subject to the length of the flat cable 9 and the dummy cables 11.
Such a transmission apparatus has the inside case 1 serving as the rotary body and the outside case 5 serving as the fixed body connected by a flat cable having conductors in its inside, so it is possible to transmit signals etc. between the rotary body and the fixed body with a high reliability. Therefore, this type of apparatus is used for the transmission of electric signals between, for example, the equipment on the body of an automobile (fixed body) and the equipment (such as air bag systems) on the steering wheel (rotary body).
In such a transmission apparatus, the winding direction inversion portion 9a is provided at the flat cable 9 and therefore it is possible to shorten the length of the flat cable to obtain the same number of rotations, as compared with winding the flat cable in one direction.
The flat cable 9 is constructed of a plurality of conductors arranged in parallel and sandwiched in by two pieces of plastic film joined by an adhesive etc. and has a suitable flexural rigidity and bending elasticity.
On the other hand, each of the dummy cables 11 is constructed of plastic tape having substantially the same flexural rigidity, bending elasticity, and external dimensions as the flat cable 9. These dummy cables 11 are provided as rotating aids so that the flat cable 9 does not jam up or slack during winding when the inside case 1 is being turned.
Note that in the above example, the explanation was made of the case of one flat cable 9 and three dummy cables 11, but the numbers of these may be suitably changed.
The state of FIG. 1 is the state with the flat cable 9 and the dummy cables 11 wound substantially to the same length at the outer circumference of the inner tubular portion 3 and at the inner circumference of the outer tubular portion 7. In other words, the winding direction inversion portions 9a and 11a are positioned at substantially half the lengths of the flat cable 9 and the dummy cables 11.
If the inside case 1 (that is, the inner tubular portion 3) is turned in the counterclockwise direction from this state, the portions of the flat cable 9 and the dummy cables 11 wound on the inside of the outer tubular portion 7 pass through the winding direction inversion portions 9a and 11a and are wound around the outer circumference of the inner tubular portion 3.
FIG. 2 shows the state of substantially all of the flat cable 9 and the dummy cables 11 wound on the inner tubular portion 3 in this way. If the inner tubular portion 3 is rotated in the counterclockwise direction from the state of FIG. 2, the result is as shown in FIG. 3. The state of FIG. 3 is referred to as "over rotation in the winding direction". In this state, the winding direction inversion portions 9a and 11a of the flat cable and the dummy cables are greatly reduced in the radius of curvature. If the inner tubular portion 3 is rotated in the counterclockwise direction from this state, the radius of curvature finally becomes substantially zero and the cables are folded back.
When this state is reached, the flat cable 9 may be creased to undergo great stress on the inside conductors and cause breakage, thereby leading up to a total transmission failure. The dummy cables 11 will not be greatly damaged, but the creases given to them may obstruct subsequent rotary operations.
To prevent this, usually the following means have been considered. FIG. 4 shows an example of a countermeasure against "over rotation in the winding direction" and provides an example of provision of a tongue shaped support piece 13 on the inside surface of the outer tubular portion 7. The support piece 13 is affixed to the outer tubular portion 7 at its base end side. Its front end gradually becomes thinner and serves as the free end. By providing this support piece 13, when the state of "over rotation in the winding direction" is reached, since a large tension is applied to the flat cable 9 and the dummy cables 11, the support piece 13 is curved as shown in FIG. 5 by this tension, so the winding direction inversion portions 9a and 11a can be prevented from being curved down to or curved less than a certain radius.
Further, since the torque required for further rotating the inner tubular portion 3 in the same direction becomes much greater than this, the state of "over rotation in the winding direction" can be detected. Therefore, it is possible to take the necessary measures to prevent more than this rotation of the inner tubular portion 3 etc.
The above explanation was made regarding the case of provision of one support piece for one dummy cable, but a plurality of support pieces may also be provided for a plurality of dummy cables.
In this way, the "over rotation in the winding direction" can be dealt with by ordinary measures. Next, an explanation will be made of the opposite "over rotation in the rewinding direction".
After the inner tubular portion 3 is rotated in the clockwise direction from the state of FIG. 1 and the flag cable 9 and the dummy cables 11 which had been wound on the inner tubular portion 3 are almost all wound back, if the inner tubular portion 3 is further rotated in the clockwise direction, the state shown in FIG. 6 is reached. This is referred to as the "over rotation in the rewinding direction".
The winding direction inversion portions 9a and 11a of the flat cable 9 and the dummy cables 11 have shorter radii of curvature in this state for the same reason as the above-mentioned "over rotation in the rewinding direction". In this state, however, the flat cable 9 and the dummy cables 11 which had been wound on the inside of the outer tubular portion 7 are pulled to the inner tubular portion 3 side and become jammed in winding. The magnitude of the force for reducing the radius of the winding direction inversion portion and the force for causing the jamming in winding change independently of each other, so the state of the reduction of the radius of curvature and jamming of winding differ depending on the balance of these forces. Whatever the case, in the end, the radii of curvature of the winding direction inversion portions 9a and 11a of the flat cable and the dummy cables 11 become substantially zero and the conductors of the flat cable are damaged.
FIG. 7 shows an example of application of the same measures as in the case of "over rotation in the winding direction" for the "over rotation in the rewinding direction". In this example, a support piece 15 is attached to the surface of the outer circumference of the inner tubular portion 3. For the support piece 15 to function in the same way as the case of the "over rotation in the winding direction", it is necessary that a large force be applied to the winding direction inversion portions 11a of the dummy cables along with the rotation of the inner tubular portion 3 in the clockwise direction and that the free end of the support piece 15 deform. But the force for pulling the flat cable 9 and the dummy cables 11 to the inner tubular portion 3 side is smaller than the force deforming the support piece 15 and therefore the flat cable 9 and the dummy cables 11 wound on the inside of the outer tubular portion 7 end up pulled to the inner tubular portion 3 side before the support piece 15 deforms much at all.
In view of the above, in the case of "over rotation in the rewinding direction", it was not possible to prevent damage to the conductors of the flat cable even with provision of the support piece 15.